The present invention relates to a coated cemented carbide cutting tool insert particularly useful for turning of cast irons, but which can also be used to cut low alloyed steels at medium to high cutting speeds. The insert has a body with WC and cubic carbonitrides as hard phases, cemented with a tough Co binder phase, and a coating with high wear resistance. The insert is produced such that the surface zone of the body is of a different elemental composition than the bulk composition, yielding good wear resistance, plastic deformation resistance and edge strength simultaneously, which results in extended tool life for different machining conditions.
Today, coated cemented carbide inserts with binder phase enriched surface zones are used for machining of steel and stainless steel materials. In these medium to coarse WC grained cutting tool materials, with relatively large additions of cubic carbonitride forming elements, the binder phase enriched surface zone widens the application area towards tougher cutting operations. However, in inserts for turning of cast irons these cemented carbide grades are often not successful. Cemented carbide grades for machining of cast iron has traditionally been designed with small WC grain size, low Co content and no or very small additions of cubic carbides, for the reason of WC grain growth inhibition only. The resulting cutting tool material has relatively high room temperature hardness, fair crack propagation resistance and bulk toughness properties. At high cutting speed and/or high feed rate operations, where large amount of heat is generated, the plastic deformation resistance and sometimes also the wear resistance is limited.
Improved resistance to plastic deformation of the cutting tool insert can be reached by even further decreasing the WC grain size and lowering the Co binder phase content, and/or by increasing the addition of cubic carbonitride forming elements. However, each of these changes will simultaneously impair the toughness properties of the insert.
Methods to improve the toughness behavior by introducing an essentially cubic carbide free and binder phase enriched surface zone are known. U.S. Pat. No. 4,277,283, U.S. Pat. No. 4,610,931 and U.S. Pat. No. 4,548,786 describe methods to accomplish binder phase enrichment in the surface region by dissolution of cubic carbide phase close to the insert surfaces. The methods require that the cubic carbide phase contains some nitrogen, since dissolution of cubic carbide phase at the sintering temperature requires a partial pressure of nitrogen, nitrogen activity, within the body being sintered exceeding the partial pressure of nitrogen within the sintering atmosphere. The nitrogen can be added through the furnace atmosphere during the sintering cycle and/or directly through the powder. The dissolution of cubic carbide phase, preferentially in the surface region, results in small volumes that will be filled with binder phase giving the desired binder phase enrichment. As a result, a surface zone consisting of essentially WC and binder phase is obtained.
U.S. Pat. No. 6,333,100 relates to a coated cemented carbide insert for turning of steels. The insert has a highly alloyed Co-binder phase, a large addition of cubic carbides from about 4 to 12, preferably from about 7 to 10, percent by weight and a WC grain size of from about 1 to 4, preferably from about 2 to 3 μm. The binder phase enriched surface zone is of a thickness <20 μm and along a line in the direction from the edge to the centre of the insert the binder phase content increases essentially monotonously until it reaches the bulk composition. The coating of the insert comprises from about 3 to 12 μm of columnar TiCN and from about 2 to 12 μm of Al2O3.
U.S. Pat. No. 5,945,207 describes a cutting tool insert particularly useful for cutting of cast iron materials. The insert is characterised by a WC-Co cemented carbide body with from about 5 to 10 wt. % Co and <0.5% cubic carbides from groups IVb, Vb or Vlb of the periodic table. The binder phase is highly W-alloyed and the surface composition is well defined. The coating comprises a layer of TiCxNyOz with columnar grains, a layer of fine-grained textured α-Al2O3 and a top layer of TiCxNyOz that has been removed along the edge line.